Light emitting diodes (LEDs) are attractive candidates for replacing conventional light sources such as incandescent lamps and fluorescent light sources. The LEDs have higher light conversion efficiencies than incandescent lamps and longer lifetimes than both types of conventional light sources. Furthermore, the conversion efficiencies of LEDs continue to improve.
An LED produces light in a relatively narrow spectral band. Hence, to produce a light source having an arbitrary color, a compound light source having multiple LEDs is typically utilized or part of the light from a single LED must be converted to light of a second wavelength, which is mixed with the light from the original LED. For example, an LED-based white light source that provides an emission that is perceived as white by a human observer can be constructed by combining light from arrays of red, blue, and green emitting LEDs that are generating the correct intensity of light at each color. Alternatively, part of the light from a blue LED can be used to excite a yellow phosphor to produce a light source that is perceived to be white.
The LEDs are typically packaged in packages that have one or more dies mounted on some form of substrate that includes power terminals for powering the dies and a heat transfer surface for removing heat from the dies. The package may also include a phosphor layer in the case of a white LED or a controller for setting the relative intensities of the colored LEDs in the case of a red, blue, and green light source that is designed to emit light over a selectable gamut of colors. The packaged LEDs are then incorporated into a final light bulb assembly or other form of luminaire that is configured to match a power source. For example, in the case of a luminaire that is to replace a conventional incandescent light source, the luminaire could include a conventional bayonet or threaded light connector that matches the conventional light sockets used by a corresponding incandescent light.
Unfortunately, the performance of an LED depends critically on the manner in which the LED is packaged. The light conversion efficiency of the LED, as well as any phosphor coating, depends on the temperature at which the LED operates and how the LED is driven. In addition, there is considerable variability from LED to LED, particularly in the case of phosphor converted LEDs, since both the variability of the LED chip and the phosphor coating can introduce variability into the performance of the final packaged LED. While the manufacturer of the packaged LEDs can “bin” the final packaged LEDs to provide products having more uniformity, the cost of such binning is significant.
Furthermore, the temperature at which the LED operates is determined by the heat sink and heat dissipating surfaces in the final luminaire. Different packaged LEDs that have the same light output at one temperature and drive current can have substantially different light output at another temperature. Hence, until the packages are assembled in the final luminaire, the extent of any such variability cannot be fully determined.
This places the burden on the luminaire manufacturer who must have the facilities needed to test the packaged LEDs in that manufacturer's luminaire, since a standardized part is not available that provides a light source with a standardized output in terms of intensity when connected to a standard AC electrical outlet. Hence, the manufacturer of the luminaire must install and maintain calibration equipment on the manufacturer's production line as well as setting the power levels for the LEDs for each light source produced so that the resulting luminaires are uniform in light output. This increases the capital investment needed to establish the production line.
Finally, even in the case of “white” LEDs, there are different color temperatures varying from “cool white” to “warm white”. Different phosphor coatings are used to create each of the color variations in phosphor-converted sources. These phosphors are normally integrated in the LED package, and hence, changing the phosphor to achieve a different color temperature requires that the LED be replaced as well. Accordingly, a luminaire manufacturer must stock different LED/phosphor sources. Similarly, the end user must change the entire luminaire to achieve a new color temperature with phosphor converted light sources. Similarly, if the LED fails, both the LED and the associated phosphor must be replaced.